A number of different life forms live on and within each fish, including pathogens. These can be the most diverse types of bacteria up to multicellular organisms. When moving, these creatures move with the fish.
Nevertheless, the fish do not suffer from an acute illness. With good husbandry conditions, the immune system of the fish can keep the number of pathogens at a level that does not affect the fish and at which they do not show any symptoms of disease.
However, a new fish usually arrives highly stressed, possibly starved, and with pathogens in tow. The importance of quarantine shows e.g. B. Numerous reports of mass deaths in aquariums after the introduction of new fish. In quarantine, the new fish should recover and gather strength.
To begin with, the water in the quarantine tank should be the same as the water the fish has been living in so far.
The water in the quarantine tank is then slowly adjusted to the values in the later aquarium. If the water you are used to is unsuitable, the water quality will of course be improved immediately.
An established and established aquarium should be used to quarantine new fish. In addition to recovery, the quarantine is also used to observe whether the new fish have an acute illness. The pool is therefore set up in such a way that the newcomers feel comfortable on the one hand, but can be observed well on the other.
Such a quarantine tank can therefore be set up almost like a normal aquarium. Only hiding places should be kept as short as possible so that the fish can be observed well.
After approx. 4 weeks at approx. 22 to 25°, most pathogens have gone through a disease cycle, ie symptoms should have become visible if the infestation was clear. The quarantine should therefore last at least 4 weeks.
Many disease cycles lengthen at cooler temperatures and accelerate at higher temperatures. As a rule, however, this will only be a really noticeable effect outside of the natural scattering at significantly different temperatures.
However, even after quarantine, fish are not free of pathogens. Even the old stock is never free of pathogens. When the old and new fish are fit, they can be put together. Their immune systems can then usually successfully fight the pathogens that were previously unknown to them.
Very careful aquarists first place one animal from the old stock with the new animals in the quarantine tank. After some time, an animal from the new stock is placed with the old stock and we wait again. Only then are all new fish placed in the target aquarium. It is questionable whether these measures really bring additional security.
Some fish species, e.g. B. discus, are partly after the new purchase in the quarantine tank treated as a precaution against various pathogens. Such preventive treatment is not recommended. Even new purchases should only be treated if they are actually ill. After a good diagnosis, they should then be treated specifically. All treatments and remedies have side effects. Problems often only arise long after the treatment and are then not associated with the agent used. Kidney damage due to treatment can e.g. B. later lead to symptoms such as abdominal dropsy. The ascites is then not attributed to the remedy, but to the sensitivity of the animals, etc. This can result in a veritable vicious circle of preventive treatments and subsequent illnesses and deaths.